Various actuators have been heretofore used for controlling a load such as a valve in accordance with an electric signal. Among these actuators, those of a construction having a driven member secured to a plunger driven by an electromagnetic force created by an electromagnetic coil are used most widely. However, in an actuator of such a conventional construction, when it is desired to drive a member to be driven in a large stroke, the axial length of the electromagnetic coil inevitably increases, thus increasing the size of the actuator and reducing the stability of the same. Furthermore, servomotors widely used for a proportional control of a driven member are formed disadvantageous when a large driving force is required, and it is difficult to maintain the driven member stably at a predetermined position by use of the servomotor.
On the other hand, the fluid pressure obtained by an electromagnetic pump is required to be maintained at a constant value, and it has been a conventional practice to detect the delivery side pressure of the electromagnetic pump, and to vary the pulse width of a pulse current based on the detected pressure for maintaining a constant pressure at the delivery side of the electromagnetic pump.
For realizing such a feature, the driving device of the electromagnetic pump has been so constructed that it comprises a monostable multivibrator driven under the control of a pulse signal generated from a self-running multivibrator, and that the time-constant of the monostable multivibrator is controlled so as to vary the pulse width of the pulse current delivered the monostable multivibrator.
However, the resistance of the coil of the electromagnetic pump is varied in accordance with the temperature rise of the coil, and therefore the current flowing through the coil is varied regardless of the constant pulse width of the pulse current supplied from the driving device so as to vary the delivery of the electromagnetic pump, such a feature constituting a drawback of the conventional construction.